1. Field of the Invention
This invention relates to a sheet transfer device used for transferring sheet materials such as recording paper sheets and image-carrying paper sheets. In particular, the present invention relates to a sheet transfer device provided with sensors for monitoring the transfer condition of the sheet materials.
2. Description of the Related Art
Sheet transfer devices are widely used in various electrical appliances, such as printers, facsimile machines and the like. Referring to FIG. 5A, a conventional sheet transfer device incorporated in a printer for example, may include a paper feed tray 2' for holding recording paper sheets 1', a feed roller 3' disposed adjacent to the paper feed tray 2', a platen roller 4' arranged in facing relation to a printhead 5' of the printer, discharge rollers 6' and a discharged paper tray 7'.
Between the paper feed tray 2' and the discharged paper tray 7' extends a paper transfer path P along which the recording paper sheet 1' is transferred when the above-mentioned rollers 3'-6' are rotated.
The conventional sheet transfer device is also provided with sensors 8'-9' for monitoring the transfer procedure of the recording paper sheet 1'. These sensors are disposed at two locations along the sheet transfer path P: at an upstream position A between the feed roller 3' and the platen roller 4', and at a downstream position B between the platen roller 4' and the discharge rollers 6'. Typically, these sensors may be mechanical devices which are arranged to generate presence signals or absence signals, depending on whether the sensors 8'-9' are held in contact with the recording paper sheet 1' or not.
The sheet transfer device having the above arrangement operates as follows. Upon actuation of the feed roller 3', a recording paper sheet 1' is pulled out of the paper feed tray 2' and moved along the transfer path P toward the printhead 5' of the printer. On its way, when the front end portion of the recording paper sheet 1' comes into contact with the upstream sensor 8', the upstream sensor generate presence signals, as previously stated. These signals are sent to a CPU (central processing unit) of the printer. Receiving the signals, the CPU begins to control the rotation of the feed roller 3' so that the front edge of the recording paper sheet 1' is properly brought to a position C between the platen roller 4' and the printhead 5'. (The operation described here is called "the paper feeding step" below.)
When the paper feeding step is finished, the printhead 5' starts printing on the recording paper sheet 1'. While the printing is performed, the recording paper sheet 1' is fed stepwise in a known manner. (The operation here is called "the printing step" below.)
When the printing step proceeds smoothly (without having the recording paper sheet 1' crumpled, for example), the front edge of the recording paper sheet 1' is brought into contact with the downstream sensor 9'. Then, the downstream sensor 9' sends presence signals to the CPU. Upon receiving the signals, the CPU determines that the sheet transfer operation is being properly performed.
Finally, when the printing step comes to an end and the rear end portion of the recording paper sheet 1' leaves the upstream sensor 8', the upstream sensor sends absence signals to the CPU. At this instant, the CPU changes the transfer mode for the recording paper sheet 1' from the currently-performed stepwise motion to a continuous discharging motion, so that the recording paper sheet 1' is swiftly sent to the discharged paper tray 7'.
The conventional sheet transfer device, however, has been found to have the following disadvantage.
As shown in FIG. 5B, the front end portion of the recording paper sheet 1' is not held between the platen roller 4' and the printhead 5' during the paper feeding step described above. In this state, the recording paper sheet 1' may unduly sag or prematurely leave the upstream sensor 8'. As a result, the absence signals are sent to the CPU of the printer, whereby the CPU, determining that the rear end portion of the recording paper sheet 1' left the upstream sensor 8', erroneously changes the transfer mode for the recording paper sheet l' to the continuous discharging motion.